Mixers are used in various communication and navigational systems to produce a radio wave of lower frequency by mixing the incoming wave with a wave generated by a local oscillator (LO). This is typically performed using the nonlinear current-voltage characteristics of diodes to act as a solid-state switch. The output of the mixer is referred to as an intermediate frequency (IF). One or more stages of amplification are typically used downstream of the mixer to increase the voltage of the IF signal. Because of this, it is important that the mixer not add an undesirable signal to the incoming wave since it would be amplified indiscriminately with the desired signal. Low noise, strong nonlinearity, repeatable electrical properties from device to device, and adequate dynamic range are important characteristics in mixer design and selection.
The useful dynamic range of a mixer is bounded by the noise level of the mixer and the level at which the mixer can no longer linearly process the incoming RF waveform, that is, when a one dB increase at the RF input port, no longer yields a one dB change at the IF output port. The dynamic range of mixers typically exceed the dynamic range of small signal amplifiers and crystal detectors. Amplifiers are thus used ahead of a mixer only where low signal level detection is critical, otherwise strong signals will degrade the RF amplifiers response and feed undesired signals to the mixer.
While mixing occurs in these devices because of the current-voltage (I-V) nonlinearity characteristic of the device, this nonlinearity also produces intermodulation distortion (IMD) products. When two closely spaced tones are mixed, for example, intermodulation products appear above and below the original pair of tones at a distance equal to the difference between the two original frequencies.
Current methods to increase the saturated output level of mixers and thereby reduce IMD products have focused on increasing the LO power level of the device by increasing the number of diodes used in the mixer circuit. This is done, for example, via multiple-diode balanced circuits, combining mixers with a 180-degree or quadrature hybrid, or another technique that splits the LO and RF input power between multiple diodes. Fabrication and matching of multiple diodes, however, is difficult and timely, thus increasing the cost of such multiple-diode mixers.
Most power semiconductor devices being utilized today are fabricated in monocrystalline silicon. However, as is known to those skilled in the art, monocrystalline silicon carbide is particularly well suited for use in semiconductor devices, and in particular for power semiconductor devices. Silicon carbide has a wide bandgap (above 3 eV), a high melting point, a low dielectric constant, a high breakdown field strength, a high thermal conductivity and a high saturation electron drift velocity compared to silicon. These characteristics would allow silicon carbide power devices to operate at higher temperatures, higher power levels, with lower specific on-resistance that conventional silicon based power devices. By way of background, a theoretical analysis of the superiority of silicon carbide devices over silicon devices is found in a publication by Bhatnagar and Baliga entitled "Comparison of 6H-SiC, and Si for Power Devices", IEEE Transactions on Electron Devices, Vol. 40, pp. 645-655, 1993. In addition to SiC, another wide bandgap material, GaN, is just becoming available. Currently GaN is being used to make blue light emitting diodes, and should prove equally effective for the same reasons as SiC.
SiC exists in a large number of polytypes which have different stacking sequences of double layers of Si and C atoms. Presently, 6H, 4H and 3C-SiC are candidate materials for SiC device fabrication. Advantages of 6H, & 4H-SiC, also known as .alpha.-SiC, are its large bandgap (approximately 3 eV) which results in high breakdown field strength. Compared to 6H, 4H-SiC, 3C-SiC, or .beta.-SiC, has a smaller Bandgap (2.2 eV) and lower maximum field strength, but has high electron mobility, like 4H. Additionally, it may be possible to grow good-quality 3C-SiC epilayers on Si which would make it a cheaper alternative to costly 6H-SiC commercial epilayers and also makes it compatible with present Si technology.